A switchgear SCV type CT can explode

[DinoChrome]

The last post in this closed thread.

http://forums.mikeholt.com/showthread.php?t=116662&page=2&

I'm a designer. I have spent most of my life in a cube. I had the pleasure and honor to have a technician/gentleman accompany me on a multi-week, multi-site project a couple years ago.

We were inspecting and cleaning MCCs and recording the results in preparation for reports to our client.

He took extra time - and pointed out the CT fuses and cautioned that it was - correct me please - very important that the fuses be replaced prior to restarting the MCC operation to avoid the CTs potentially exploding.

I'd like to propose adding this to the Safety sticky at the top of this forum.

(I could use some help putting it into layman's terms as to why this occurs.)

Thanks,
Bill

 

[don_resqcapt19]

As far as I know you don't use fuses on CTs. An open circuit on a CT can result in catastrophic failure of the CT. If the fuse would blow, you would have an open circuit.

 

[big john]

Fusing a CT is literally just about the last thing someone would want to do.

1) If this powers circuit protection, an open fuse would prevent that protection from opening the circuit, creating a fire hazard.
2) Since CTs are fundamentally a type of step-up transformer, if you open the secondary under load you develop extremely high voltages that at best will destroy the CT, and at worst can cause a lot of equipment damage or start fires.
3) Because CTs are generally inherently current limited by the primary circuit impedance, it's very unlikley one will be in a sustained overload condition, making fusing unnecessary.

Now, that said potential transformers do have fuses. And PT sounds a like CT, so maybe this is just someone confused about their acronyms?

 

[Jraef]

Maybe he meant "replaced" as in "replaced, with a shorting bar"!

On one occasion years ago, I saw where someone likely misinterpreted what the shorting terminal block was or was for and managed to get fuses in there. They were the stud type shorting blocks and although the studs didn't come up over the shorting bar, the guy made little copper Z brackets that went over each stud, then up above the shorting bar, then out to where he could bolt on some blade type fuses. A lot of clever work for something he should not have done in the first place.